Blistering

by Tony
(Barrow in Furness)

Paul. Last couple of months have seen me been plastering quite a bit in our renovation project. I have been having great results and I believe this is through understanding each stage and me being in control of the plaster and not the other way round.

My last couple of walls though have presented something I hadn't previously encountered and wondered if you could offer any advice.

I'm noticing what appear to be small blisters (grouped together) on my first coat and no matter what I try, I cannot flatten them before applying the second coat.

Some I can finally get rid of, but some remain which I am considering sanding down.

Walls are pre plastered, but I have tended to use Plasprime as I prefer this to using PVA.

I'm not doing anything different (that I am aware of) from the walls which have turned out blemish free.

Regards,Tony

AnswerHi Tony,Blisters can cause much annoyance and frustration - so the best way to deal with them is to ensure you leave it until the time is right. Then they are easily eliminated.

Ignore Blisters until the time is right otherwise you can make them worse.If they appear in the 1st coat they can be held back there simply by letting the plaster firm up a bit more. But if the 2nd coat is applied too quickly they will come through and affect this 2nd coat of plaster. Leaving the 1st coat until it is really quite firm before you move onto stage 2 (the flattening stage) prior to applying the 2nd coat should deal with them. Understanding blistersBlisters are just bubbles of air trapped beneath the plaster. When the trowel runs over the blisters they will, if the plaster is quite firm, pop. However if the plaster is quite wet and pliable they will not pop and simply just move around.

Knowing whether to deal with blisters in the 1st coat or if to leave?If the whole wall is covered in blisters then it is best to wait until the plaster is quite firm so that they can be removed. If the majority of the wall is okay and just a few blisters exist then it is best to go with the majority if it is ready and start moving on. The blisters can then be dealt with in the 2nd coat.

Blisters in the 2nd coatIf blisters appear in the 2nd coat of plaster, then just ignore them, and whatever you do don't try and get them out - just carry on through all the trowelling stages and complete the necessary objectives, such as filling hollows and removing trowel marks. You may well find the blisters improve, but if not don't worry. It is then purely a matter of letting the plaster firm up. When the plaster is firm enough you'll be able to run your trowel over them and they'll pop, the plaster will still be pliable enough to smooth the tear mark. There is no way of knowing exactly when the time is right - it is just a case of having a go, if the blisters do not come out then wait a few more minutes and try again. Sometimes you need to leave it right up until the plaster is nearly set.

In summaryBlisters are best ignored until the plaster is really quite firm. When it is they will pop and can be smoothed over. It is not uncommon for the wall to be looking good in every way apart from blisters and have to go back to it 30 minutes later with the sole purpose of removing them.